to mark the exact place before shooting the marble or ball by putting one's knuckles at this spot.

Sentence 1

Please knuckle down before you shoot that marble.

Meaning 2 ►▼

to work hard

Sentence 2

Knuckle down to work. You're talking too much.

Derivation

From the game of marbles: a player had to "knuckle down"by another player if he moved his marble before shooting it. The following are some interesting idioms which are probably related to cards and gambling, but it is difficult to be sure of the derivation.